In data processing systems, magnetic disc drives are often used as direct access storage devices. In such devices, read/write heads are used to write data on or read data from an adjacently rotating hard or flexible disk. To prevent damage to either the disc or the read/write head, it has been recognized that the surface of the disc should be very flat and free of any bumps or the like which might be contacted by the read/write head. Also, the read/write heads have been designed so that they will fly over the surface of the rotating disc with a small fly height which is maintained by a film of air. During its flight, the head undergoes continuous vibration, pitch and roll as the topography of the disc changes beneath the head. If the quality of the disc or the read/write head is poor, occasional rubbing or sharp contact may occur between the disc and the read/write head. Such contact may damage the head or the disc, cause loss of valuable data, or all of these.
In order to certify that a magnetic disc is adequately smooth for use in a disc drive system, glide height tests must be performed on the disc. Glide height testing is used to verify that a magnetic recording disc is able to accommodate a specified glide height. As the density of data recorded on magnetic discs continues to increase, the flying height or glide height of magnetic transducers with respect to the disc must be reduced to accurately read and write information on the disc. As a result, the magnetic recording disc must accommodate the lower glide height of the transducer and the slider supporting it, meaning that the disc surface must be extremely smooth and uniform.
Since glide head technology relies on direct contact with a defect in order to function, the range of detectable defect sizes (or defect bandwidth) is limited by the design of the probing structure. Glide heads designed to detect large defects (macro-defects) generally can not detect nano-defects, while those designed to detect nano-defects are at risk of damage by large, catastrophic defects. In addition, more stringent requirements exist for testing the extreme inner and outer radii of the media. Probing technology must be able to function on these areas as well.
Media certification designs incorporate a wide contact area on the air bearing topographical surface. The detection pad may be minimally on the order of 8 to 10 mils wide in order to accommodate an adequate track step size, which is required for production throughput. If multiple contact pads are integrated into the design for inner diameter/outer diameter access, then there is no way to determine which pad is excited, and thus the defect event can be anywhere within the entire slider width. This leaves great uncertainty as to the precise location of defects, and can result in large areas of the media being labeled as defect zones rather than precise locations. This leads to lower data storage capacity of the media than may in fact be available.
There is also a physical limit to glide head capability to resolve smaller defects. Although the surface is wide and durable, very small defects fail to significantly induce slider body modes required to excite ex situ or sensor attached configurations. As the requirement to detect micro and nano-defects comes into play, these even become more of an issue.